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Organoselenium chemistry
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<p>Organoselenium chemistry is the science exploring the properties and reactivity of organoselenium compounds, <a href="/facts/Chemical_compound/37FXqqtv">chemical compounds</a> containing <a href="/facts/Carbon/ukrgQexo">carbon</a>-to-<a href="/facts/Selenium/k975NApC">selenium</a> <a href="/facts/Chemical_bond/ArNUMnIB">chemical bonds</a>. Selenium belongs with oxygen and sulfur to the <a href="/facts/Group_16_element/nXLGmnqL">group 16 elements</a> or chalcogens, and similarities in chemistry are to be expected. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments. </p><p>Selenium can exist with <a href="/facts/Oxidation_state/W53W6s6V">oxidation state</a> −2, +2, +4, +6. Se(II) is the dominant form in organoselenium chemistry. Down the group 16 column, the <a href="/facts/Bond_strength/q4be3EeR">bond strength</a> becomes increasingly weaker (234 <a href="/facts/Kilojoule/9ndq9jD2">kJ</a>/<a href="/facts/Mole_(unit)/kCMFwCc0">mol</a> for the C−Se bond and 272 kJ/mol for the C−S bond) and the <a href="/facts/Bond_length/rHHIBH6q">bond lengths</a> longer (C−Se 198 pm, C−S 181 pm and C−O 141 pm). Selenium compounds are more <a href="/facts/Nucleophilic/xYccoaJQ">nucleophilic</a> than the corresponding sulfur compounds and also more acidic. The <a href="/facts/PKa/t4YyRtKC">p<i>K</i>a</a> values of XH2 are 16 for oxygen, 7 for sulfur and 3.8 for selenium. In contrast to <a href="/facts/Sulfoxide/Stk3Vnj9">sulfoxides</a>, the corresponding selenoxides are unstable in the presence of β-protons and this property is utilized in many <a href="/facts/Organic_reaction/SESmeGWm">organic reactions</a> of selenium, notably in selenoxide oxidations and in selenoxide eliminations. </p><p>The first organoselenium compound to be isolated was <a href="/facts/Diethyl_selenide/b30MS5XP">diethyl selenide</a> in 1836. </p>